Differences in plant cell wall composition account for much of the variation in chemical reactivity, mechanical strength, and energy content of plant material. In turn, differences in chemical and mechanical properties of plant material greatly impact the utilization of plant biomass by agriculture and industry. One abundant component of many types of plant cells, and one which has garnered increasing attention because of its importance in plant utilization, are lignins.
Lignins are a class of complex heterpolymers associated with the polysaccharide components of the wall in specific plant cells. Lignins play an essential role in providing rigidity, compressive strength, and structural support to plant tissues. They also render cell walls hydrophobic allowing the conduction of water and solutes. Reflecting their importance, lignins represent the second most abundant organic compound on Earth after cellulose accounting for approximately 25% of plant biomass. Lignins result from the oxidative coupling of three monomers: coumaryl, coniferyl, and sinapyl alcohols. Variability in lignin structure is dependent, in part, upon the relative proportion of the three constitutive monomers.
The biosynthesis of lignins proceeds from phenylalanine through the phenylpropanoid pathway to the cinnamoyl CoAs which are the general precursors of a wide range of phenolic compounds. The enzymes involved in this pathway are phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-coumarate-3-hydroxylase (C3H), O-methyltransferase (OMT), ferulate-5-hydroxylase (F5H), caffeoyl-CoA 3-O-methyltransferase (CCoA-OMT), and 4-coumarate:CoA ligase (4CL). Whetten and Sederoff, The Plant Cell, 7: 1001–1013 (1995); Boudet and Grima-Pettenati, Molecular Breeding, 2:25–39 (1996).
The lignin specific pathway channels cinnamoyl CoAs towards the synthesis of monolignols and lignins. This pathway involves two reductive enzymes that convert the hydroxycinnamoyl-CoA esters into monolignols: cinnamoyl-CoA reductase (CCR), and cinnamyl alcohol dehydrogenase (CAD).
While lignins are a vital component in terrestrial vascular plants, they often pose an obstacle to the utilization of plant biomass. For example, in the pulp and paper industry lignins have to be separated from cellulose by an expensive and polluting process. Lignin content also limits the digestability of crops consumed by livestock. While reduction of lignin content for such applications is generally desirable, increasing lignin content in plant material intended as a chemical feedstock for production of phenolics, for use as a fuel source, or for improvement in agronomically desirable properties (e.g., standability) is also advantageous. Accordingly, what is needed in the art is the ability to modulate lignin content in plants. The present invention addresses these and other needs.